Polycrystalline diamond is used in many demanding and abrasive applications such as oil well drilling and hard materials machining. Its superior mechanical properties and resistance to abrasion are suitable for bearing components in medical implant devices, such as artificial joints. One of the main obstacles in using PCD (polycrystalline diamond) in medical implant devices is that normal materials and processes to create PCD produce a material that is not biocompatible.
PCD is fabricated by subjecting unbonded diamond powder to extreme pressure and heat in the presence of a solvent metal. The powder is often placed adjacent to a substrate and surrounded by a refractory metal containment can. In some cases, a refractory metal layer is placed between the substrate and the inner diamond layer. The assembly (containment can, unbonded diamond powder and solvent metal, and the substrate) is placed in a high pressure cell and pressurized in a hydraulic press to more than 55 Kbar. The mixture is then heated to a temperature above the melting point of the solvent metal, at which point the solvent metal melts and flows or sweeps into the interstitial voids between adjacent diamond crystals. The solvent metal is driven by the pressure gradient to fill the voids.
Carbon atoms from the surface of the diamond crystals dissolve into the molten solvent metal, forming a carbon solution. When the proper temperature and pressure are reached, diamond formation is thermodynamically favored and the carbon that is held in solution in the molten solvent metal crystallizes out onto the diamond grains, bonding adjacent diamond grains together with diamond to diamond bonds. This forms a sintered polycrystalline diamond structure with the solvent metal in the interstitial spaces between the diamond grains. In addition to the crystallization of the dissolved diamond to bond adjacent diamond crystals, the dissolved carbon may react with the solvent metal to form metal carbides.
The diamond in the resulting sintered compact is highly inert and biocompatible. Thus, the exposed non-diamond constituents of the polycrystalline diamond compact such as the interstitial solvent metal in the PCD are what makes the PCD biocompatible or not.
One of the tests that is used to determine the biocompatibility of PCD, or other materials, is an elution test. In this test a PCD part or a portion thereof is placed in a container with a solution that is used to simulate body fluid. For this purpose, Hank's Balanced Salt Solution (HBSS) is often used as the solution for the elution test. Hank's solution contains a mix of salts and phosphates. A phosphate buffer can be added to stabilize the pH of the solution at a desired value. A controlled amount of solution is placed with the PCD part for a prescribed period of time, usually 24 hours. The amount and type of material that is released by the PCD and into the fluid through corrosion processes are measured, typically by Inductively Coupled Plasma Mass Spectroscopy (ICPMS).
Normal body pH is 7.4, but this pH fluctuates over time and can be significantly lower in local areas. In hematomas, where circulation in impaired, the pH may dip as low as 6 for a short period of time. For implants in the immediate post surgical time period, a hematoma like condition can exist around the implantation site. Thus, pH 6 is a worst case environmental condition that serves as an appropriate test case for corrosion resistance and elution testing. There is another advantage to using pH 6 as the elution test environment. For materials that are sensitive to pH, either due to natural material chemistry, electrocorrosion, crevice corrosion, or pitting mechanisms, testing under a slightly more acidic pH will show problems quickly that would take much longer tests in pH 7.4 to adequately demonstrate. Since implants are generally in place for many years, and hopefully the remainder of the patient's life, long term corrosion resistance is an essential part of implant material biocompatibility. It is for these reasons that elution tests are preferentially conducted at pH 6.
Release of metallic ions from medical devices is a long term concern that is well documented in the medical literature. Elevated serum metal ion concentrations are present in patients that have metal on metal hip and spine arthroplasty devices. The long term effect of these elevated levels is unknown, but potential increased risk for cancer and other malignancies remote from the implantation site are a real concern. Local acute toxicity effects of these devices can be observed in tissue directly surrounding these devices. Studies in the literature for patients with metal on metal hip implants report an increase in serum and urine Co and Cr levels of between 3 and 23 times relative to normal levels.
Some references indicating these elevated levels of Co and Cr are: Skipor, Anastasia, Pat Campbell, et al. Metal Ion Levels in Patients with Metal on Metal Hip Replacements. Society for Biomaterials 28th Meeting Transactions, 2002, and Josh Jacobs et al. Cobalt and Chromium Concentrations in Patients with Metal on Metal Total Hip Replacements. Clinical Orthopedics, S256-S263, 1996.
The traditional metals used in sintering diamond to make PCD are the first row transition metals in the periodic table. These most notably being cobalt, but also manganese, iron, and nickel for example. None of these metals are both corrosion resistant and biocompatible by themselves. Typically, metals may be made more corrosion resistant by adding elements that form stable oxide films to the metal. Chrome is the most notable element for this purpose. Chrome is added to steels to create stainless steels and it is added to cobalt to form biocompatible alloys used extensively in orthopedic implants such as CoCrMo ASTM F-75 or ASTM F-799.
It has been discovered that attempts to use alloys that contain chrome as a sintering metal to form PCD did not work as effectively as intended, and often resulted in PCD which was not fully biocompatible, as it suffered from corrosion and ion elution. One reason that these diamond compacts were not fully corrosion resistant is that chrome is a strong carbide former. During the sintering process, the chrome is exposed to the dissolved carbon which is held in solution in the molten solvent metal. When this occurs, chrome will precipitate out of the molten metal as chrome carbide. This leaves the original solvent metal, cobalt for example, depleted or devoid of chrome in some areas. This creates a PCD which has areas of exposed metal at the surface which are depleted in chrome and therefore with reduced corrosion protection or biocompatibility. It is thus appreciated that the attempts to find a solvent metal for sintering diamond into a biocompatible compact have been hindered by the reactions occurring between the metal and the diamond during sintering. The interstitial metal and carbides in the resulting sintered diamond compact are quite different than the solvent metals used as a starting material. As a result, metals which have good biocompatibility by themselves have proven to have poor biocompatibility after being used as a solvent metal in forming a sintered diamond compact.